Electric circuits for supplying a substantially constant current to a load



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inl'vsniroil J. P. CLARKE 3,297,880 ELECTRIC CIRCUITS FOR SUPPLYING ASUBSTANTIALLY Jan. 10, 1967 CONSTANT CURRENT TO A LOAD Filed May 9, 19633 Sheets-Sheet 2 mveu'ro (/Omv Z/LIP (ZFWKF Fig.2

Jan. 10, 1967 Filed May 9, 1963 J. P. CLARKE ELECTRIC CIRCUITS FORSUPPLYING A SUBSTANTIALLY CONSTANT CURRENT TO A LOAD 5 Sheets-Sheet 3Fig.3

Alarm F TTQRNEYS United States Patent Ofifice 3,297,880 Patented Jan.10, 1967 3,297,880 ELECTRIC cmctn'rs non SUPPLYING A son- STANTIALLYCGNSTANT CURRENT TO A LOAD This invention relates to electric circuitsfor supplying a substantially constant current to a load.

According to the present invention, an electric circuit for supplying asubstantially constant current to a load comprises means arranged tosupply a control signal the value of which depends upon the value of thecurrent supplied to the load, an oscillator to which the control signalis supplied, the oscillator being arranged to supply an oscillatorysignal the frequency of which varies with variations in the controlsignal, a network to which the oscillatory signal is supplied, thenetwork being such that the value of the current passed by the networkvaries with variations in the frequency of the oscillatory signal, and arectifier arrangement which is arranged to rectify the signal suppliedby said network, the current passed by the rectifier arrangementproviding the required current which is supplied to the load, thearrangement being such that a change in the value of the currentsupplied to the load results in a change in the frequency of theoscillatory signal such that the value of the current passed by saidnetwork changes in such a sense as to tend to restore the currentsupplied to the load to the required value.

The control signal may be derived from across a resistor which isarranged to be connected in series with the load.

An electric supply system in accordance with the present invention forsupplying direct current of substantially constant value to a balancedload, will now be described by way of example with reference to theaccompanying drawings, of which:

FIGURE 1 shows a block schematic representation of the system, and

FIGURES 2 and 3 together show diagrammatically a part of the system ofFIGURE 1 when the two figures are placed side-by-side with FIGURE 2 tothe left of FIG- URE 3.

Referring first to FIGURE 1, the system comprises two parts 1 and 2,each of which acts as a source of substantially constant current. Sincethe parts 1 and 2 are substantially similar only the part 1 will bedescribed in detail.

The part 1 includes a multivibrator 3 which supplies an oscillatorysignal by way of a low pass filter 4 to a power amplifier 5, the outputsignal of which passes to a rectifying and smoothing network 6. Thepositive output lead 7 of the network 6 is connected to the negativelead 8 of the part 2 and is also connected by way of a resistor 9 toearth, and the negative output lead 10 of the network 6 is connected toa terminal 11.

A control signal that is dependent upon the value of the current carriedby the lead 7 is passed to a control amplifier 12 the output of which isutilised to control the frequency of oscillation of the multivibrator 3.

During operation the multivibrator 3 supplies an oscillatory signalwhich normally has a frequency of about five kilo'cycles per second, butwhich may be varied over a range of a hundred cycles per second oneither side of that frequency in dependence upon the signal supplied bythe control amplifier 12. The characteristic of the filter 4 is suchthat the amplitude of the oscillatory signal passed to the poweramplifier 5 depends upon the frequency of the oscillatory signal. Thusthe amplitude of the output signal from the amplifier 5, and hence thevalue of the current passing through the leads 7 and 10 depends upon thefrequency of the oscillatory signal supplied by the multivibrator 3.

If the current flowing in the lead 7 departs from the required value thesignal passed to the control amplifier 12 varies from its normal value,and the consequent variation in the output signal of the controlamplifier 12 causes the frequency of oscillation of the multivibrator 3to vary in such a sense as to tend to restore the current flowing in thelead 7 to the required value.

The characteristic of the filter 4 is such that the normal frequency ofoscillation of the multivibrator 3 falls at the mid-point of itstransistion range, and such that over a range of frequencies from ahundred cycles per second below the normal frequency to a hundred cyclesper second above the normal frequency the attenuation produced by thefilter 4 increases linearly with frequency.

Referring now to FIGURES 2 and 3, in which the circuit of the part 1 ofFIGURE 1 is shown diagrammatically, the multivibrator 3 comprises a pairof junction transistors 13 and 14 which have their collector and baseelectrodes cross-coupled in conventional manner by way of capacitors 15and 16. The base electrodes of the transistors 13 and 14 are furtherconnected by way of resistors 17 and 18 respectively to a point 19 thepotential of which is determined in a manner to be describedhereinafter.

A resistor 20 which forms part of the load impedance of the transistor14 is connected by way of leads 21 and 22 between the input terminals 23and 24 of a low pass filter 4. The output terminals 25 and 26 of thefilter 4 are connected to the primary winding 27 of a transformer 28,the secondary winding 29 of which is connected in the input circuit ofthe power amplifier 5. The power amplifier 5 comprises a transistoramplifier stage 30 which acts as a driver amplifier and phase-splitterfor a pushpull amplifier stage 31.

The secondary winding 32 of the output transformer 33 of the amplifierstage 31 is connected across one diagonal of a bridge rectifier 34, theother diagonal of which is connected to the load circuit by way of theleads 7 and 10. Capacitors 35 and 36 connected between the leads 7 and10 and resistors 37, 38 and 39 in series with the lead 7 form asmoothing network for the output of the bridge rectifier 34.

The terminals of the resistor 38 are connected, by way of leads 41) and41, to the input circuit of the control amplifier 12, and the terminalsof the resistor 39 are connected to the input of an alarm circuit 42.

The control amplifier 12 comprises a transistor 43, connected in thecommon emitter configuration, having its collector electrode connectedby way of a resistor 44 to the base electrode of a transistor 45 whichis connected as an emitter-follower. The emitter electrode of thetransistor 45 is connected to the point 19 in the multivibrator 3.

The lead 41 is connected to the junction 46 between a resistor 47 and aZener diode 48 which are connected, in series with a diode 49, betweenthe terminals of a further Zener diode 5h. The junction 46 is thusmaintained at a substantially constant negative potential with respectto the positive supply line 51. The lead 40 is connected to the baseelectrode of the transistor 43. The positive and negative supply lines51 and 52 from the power supply 53 are isolated from earth potential, sothat the control amplifier 12, the multivibrator 3 and the filter 4 havea potential with respect to earth which is determined by the potentialon the lead 41.

In operation the voltage applied between the base electrode of thetransistor 43 and the supply line 51 in the control amplifier 12 isequal to the sum of the voltages developed across the Zener diode 1$,the diode 49 and the resistor 38 which is in series with the output lead7. Under normal operating conditions a negative bias with respect to thesupply line 51 is thus applied to the base electrode of the transistor43, the magnitude of this bias increasing when the voltage across theresistor 38 de- CIGESCS.

The negative bias on the base of transistor 43 determines the biasapplied to the base electrode of the transistor 45 and hence determinesthe potential at the point 19. The potential at the point 19 determinesthe rate of discharge of the capacitors 15 and 16 in the multivibrator 3and hence the frequency of oscillation of the multivibrator 3.

If the current supplied to the load falls below the required value, thevoltage developed across the resistor 38 decreases in magnitude so thatthe bias on the base electrode of transistor 43 becomes more negative.Hence the potential at the point 19 becomes less negative with respectto the supply line 51, the capacitors 15 and 16 in the multivibrator 3discharge more slowly, and the frequency of oscillation is lowered. Dueto the characteristic of the low-pass filter 4 the amplitude of thesignal passed to the power amplifier 5 is greater at the lowerfrequency, so that the output of the amplifier 5 is increased, thisincrease tending to restore the current in the load circuit to therequired value.

If the current in the load circuit increases the frequency ofoscillation of the multivibrator 3 increases, so that the output of theamplifier 5 is reduced. The required stabilisation of the value of thecurrent supplied to the load is therefore achieved.

The part 2 shown schematically in FIGURE 1 differs from the part 1described above only in the respect that the normal frequency ofoscillation employed in the part 2 is about five and a half kilocyclesper second, the filter 54 in the part 2 having a characteristic suitablefor operation, in the manner described above, at this frequency. Thedifferent operating frequencies are employed in order to avoid thepossibility of any coupling between the two parts 1 and 2 resulting inlow frequency beats which would be diflicult to filter out of therectified output of the circuit.

It will be noted that the output paths of the parts 1 and 2 areconnected in series to provide supplies at terminals 11 and 55respectively negative and positive with respect to earth potential. Inorder that the two sources of substantially constant current may be soconnected in series the performance of the part 2 is deliberatelydegraded by the addition of a shunt resistor 56 connected between theterminal and the lead 8 Although the system described above withreference to the accompanying drawings comprises the two substantiallysimilar parts 1 and 2, it will be appreciated that either of theseparts, for example the part 1 described with reference to FIGURES 2 and3, may be used independently as a source of substantially constantcurrent.

I claim:

1. An electric circuit for supplying a substantially constantunidirectional current to a load, said circuit compris- (I) analternating current generator, the frequency of which is variable over apredetermined range,

(II) a passive network having a frequency response that varies linearlyat least over said range of frequencies,

(III) a power amplifier,

(IV) means connecting the output of the generator to the input of saidamplifier by way of said network,

(V) rectifier means to derive from the output signal of the amplifier,the unidirectional current which is to be passed to the load,

(VI) means to derive a unidirectional control voltage from saidunidirectional current, and

(VII) means to supply the control voltage to said generator to controlthe frequency thereof.

2. An electric circuit according to claim 1 wherein the means to derivesaid control voltage includes a resistor connected in series with theload.

3. An electric circuit according to claim 1 wherein the network is alow-pass filter network.

4. An electric circuit according to claim 1 wherein the oscillator is amultivibrator.

5. An electric supply system for supplying direct current to a balancedload comprising two electric circuits, each of which is in accordancewith claim 1, and means connecting the output paths of said two circuitsin series across the balanced load.

References Cited by the Examiner UNITED STATES PATENTS 3,010,078 11/1961Stefanov 331-113 X 3,183,432 5/1965 Pintell 3,197,691 7/1965 Gilbert3218 JOHN F. COUCH, Primary Examiner.

W. H. BEHA, Assistant Examiner.

1. AN ELECTRIC CIRCUIT FOR SUPPLYING A SUBSTANTIALLY CONSTANTUNIDIRECTIONAL CURRENT TO A LOAD, SAID CIRCUIT COMPRISING (I) ANALTERNATING CURRENT GENERATOR, THE FREQUENCY OF WHICH IS VARIABLE OVER APREDETERMINED RANGE, (II) A PASSIVE NETWORK HAVING A FREQUENCY RESPONSETHAT VARIES LINEARLY AT LEAST OVER SAID RANGE OF FREQUENCIES, (III) APOWER AMPLIFIER, (IV) MEANS CONNECTING THE OUTPUT OF THE GENERATOR TOTHE INPUT OF SAID AMPLIFIER BY WAY OF SAID NETWORK, (V) RECTIFIER MEANSTO DERIVE FROM THE OUTPUT SIGNAL OF THE AMPLIFIER, THE UNIDIRECTIONALCURRENT WHICH IS TO BE PASSED TO THE LOAD, (VI) MEANS TO DERIVE AUNIDIRECTIONAL CONTROL VOLTAGE FROM SAID UNIDIRECTIONAL CURRENT, AND(VII) MEANS TO SUPPLY THE CONTROL VOLTAGE TO SAID GENERATOR TO CONTROLTHE FREQUENCY THEREOF.